In recent years, the development of laser is remarkable. Particularly, with respect to solid lasers and semi-conductor lasers having a light emission region from near infrared rays to infrared rays, those having a high output and a small size become readily available. As an exposure light source during the plate-making directly from digital data from computers are very useful these lasers.
A positive-working lithographic printing plate precursor for infrared laser contains an alkaline aqueous solution-soluble binder resin and an infrared ray absorbing dye for absorbing light to generate heat (light-heat converting substance) and so on as essential components. In an unexposed area (image portion), the infrared ray absorbing dye and so on function as a dissolution inhibitor to substantially lower the solubility of the binder resin by the mutual action with the binder resin; and in an exposed area (non-image portion), the mutual action between the infrared ray absorbing dye and so on and the binder resin becomes weak by the generated heat, and the infrared ray absorbing dye and so on are dissolved in an alkaline developing solution, to form a lithographic printing plate.
However, in such a positive-working lithographic printing plate precursor for infrared laser, even in the case where the surface state slightly changes by, for example, touch on the surface thereof during the treatment, the unexposed area (image portion) is dissolved to form scars during the development, resulting in problems such as deterioration in printing resistance and poor ink acceptability.
Further, in the case of a negative-working lithographic printing plate precursor for infrared laser, there was a problem such that the scratch resistance of the recording layer in an unexposed state before curing is insufficient.
As means for solving the above-described problems, for example, U.S. Pat. No. 6,124,425 discloses examples of an alkali-soluble resin having an infrared ray absorbing functional group in the side chains thereof for the purpose of simply achieving the film strength (image strength). That is, it is intended to enhance the film strength by introducing a partial structure having a light-heat converting function into an alkali-soluble resin to reduce the components in the material. However, since the alkali-soluble resin is a polymer compound having a molecular weight of 5,000 or more, not only the adhesiveness to the support increases, but also the solubility in the processing agent during the development is insufficient. In particular, in the case where the alkali-soluble resin is used as a positive-working lithographic printing plate material, the solubility of the non-image portion is low, and the recording layer that should be removed is not sufficiently removed but becomes a residul film, resulting in a problem that the non-image portion is likely stained.
Further, it is known in JP-A-2000-35666(the term “JP-A” as used herein means an “unexamined published Japanese patent application”) that the addition of a low-molecular weight wax enhances the surface slipperiness and realizes superior scratch resistance. However, since the wax has a low molecular weight, there are problems such as transfer of the wax to a protective paper (laminated paper) or the back surface of the support during the lamination of a lithographic printing plate precursor, and transfer of the wax to rollers during the manufacture of a lithographic printing plate precursor, leading to unstable factors during the manufacture or conveying.
Moreover, European Patent Nos. 950,514 and 950,517 propose examples of realizing the slipperiness by the addition of a polysiloxane-based surfactant. However, for the possibility of generation of scum and difficulty in controlling the slipperiness such as causing excessive slipping, there were unstable factors during the manufacture or conveying, too.
In addition, it may be considered to provide a protective layer on the recording layer. However, for example, in the case of providing a general protective layer using an aqueous resin, especially when used under a high humidity condition, the protective layer adheres to the support and hardly peels apart therefrom, resulting in lowering in the workability. In any means, the productivity was poor.
For these reasons, it has been demanded to realize a lithographic printing plate precursor that does not lower the workability, does not affect the image-forming properties, and can inhibit scars of the recording layer.